Microstructure And Distortion Of Niobium-containing 18CrNiMo7-6 Gear Steel After High-temperature Carburization

High-temperature carburizing has been applied more and more widely as the surface hardening method for gears in place of ordinary gas carburizing due to its high efficiency,low cost and low gas emission.It is crucial to prevent austenite abnormal coarsening and heat treatment distorsion for high-temperature carburizing,which can not be avoided for ordinary case hardening steels.In this work,the case hardening steel18 CrNiMo7-6,which is used for wind power gears,together with the addition of Nb in the range of 0.03?0.10%,was taken as the experimental steels.The austenite coarsening behavior,the microstructure and mechanical properties after high-temperature carburizing,and heat treatment distortion after 980 ? carburizing and quenching were investigated for the experimental steels by means of the finite element method and the experimental methods such as pseudo-carburizing and high-temperature carburizing.The results of this work could be helpful for the development of new materials suitable for high-temperature carburizing.Results of austenite grain growth of the experimental steels showed that the addition of Nb in the content of 0.03% and above could evidently prevent grain coarsening trend due to the pinning effect of Nb C precipitates.The activation energy for austenite grain growth was increased by 50 k J/mol if the Nb content of 0.03% was added,and a further increase in Nb content could not evidently increase the activation energy.The experimental steel with 0.03% Nb had an average prior austenite grain size of Grade No.9 when it was carburized at 980 ? for 37 h with an effective hardening depth of about 4 mm,which could meet the requirement of wind power gears in the aspect of fine grain size.Results of microstructure and mechanical properties of the experimental steels after high-temperature carburizing showed that the experimental steel with 0.03% Nb had high-40 ? impact toughness of more than 80 J at the tensile strength level of 1400 MPa.The retained austenite fraction at the location of 1 mm in depth in the carburized case was measured to be about 20% after high-temperature carburizing.The nanohardness values were 11.5 GPa and 8.0 GPa in the case and in the core,respectively.The nano-hardness was well correlated to the Vickers hardness,indicating that the retained austenite in the high-temperature carburized case was fine and could not apparently influence the nano-hardness.Results of numerical simulation for the Navy-C specimens showed that the interaction of thermal stress and phase transformation stress resulted in variaition of the opening displacement in a trend of first increasing,then decreasing and finnally increasing.The opening displacement was minimal when the martensite fraction in the core was about 50%.The experimental steel with 0.03% Nb had a lower opening displacement due to its lower and more uniformly distributed residual stress,which could contribute to its lower heat treatment distortion in comparision with the experimental steel 18 CrNiMo7-6 without Nb addition.Experimental results showed that the measured opening displacement of the 0.03% Nb experimental steel was lower than that of the experimental steel 18 CrNiMo7-6 without Nb addition.Thus,the distortion of gears after high-temperature carburizing and quenching could be mitigated by means of Nb microalloying as a result of decreased residual stresses.In summary,the microstructure and mechanical properties of Nb-microalloyed18 CrNiMo7-6 gear steel after 980 ? high-temperature carburizing could meet the requirements of large gears for wind power applications,and the heat treatment distortion was also reduced evidently.Thus,the Nb-microalloyed 18 CrNiMo7-6 steel could be used as the gear steel for 980 ? high-temperature carburizing.